A fatigue phenomenon of metal is conventionally known that stresses are repeatedly applied to metal and micro crack (fatigue crack) initiates, even if the stresses are much lower than ultimate tensile strength. By further applying stresses repeatedly to the portion where the fatigue crack has initiated, the fatigue crack grows and leads to failure of metal. On the other hand, in a society at these days, metal is used everywhere. Accordingly, restraint of fatigue crack growth and early detection of fatigue crack are very important issues.
FIGS. 8A-8D are diagrams for explanation of initiation and growth of fatigue crack. As shown in FIG. 8A, micro crack initiates in a metal surface due to accumulation of fatigue damage by repeated stresses. Further, as shown in FIG. 8B, the crack opens by application of tensile load, and, as shown in FIG. 8C, the crack closes by removal of the tensile load. Furthermore, as shown in FIG. 8D, the crack opens by reapplication of tensile load. Thus, the crack propagates by repeating the opening and closing of the crack. Here, there is a problem that it is difficult to visually detect crack because the crack closes when no tensile load is applied as shown in FIGS. 8A and 8C.
As a related technology, Japanese Patent Application Publication JP-A-8-29410 (pp. 1-3 and FIG. 1) discloses a crack detection method of nondestructively detecting crack, which occurs randomly with respect to portions and orientations, at a high speed with high sensitivity. The method is usable in production lines for solid objects having black opaque appearance, rough surfaces and various shapes.
According to the crack detection method, the presence of crack can be easily detected by impregnating the crack with a volatile solvent such as acetone and benzene, leaving the sample with a dried surface in a sealed container at rest, and detecting the concentration of the volatile solvent that has vaporized and mixed in a carrier gas such as helium or nitrogen flowing along the surface of the sample.
However, in the crack detection method, even if the presence of crack has been detected, the portion where the crack of the sample is present cannot be detected. Further, since the sealed container is required, the detection is difficult with respect to a large-scaled sample such as a ship or aircraft. Therefore, in order to detect the portion where the crack of the sample is present, it is necessary to employ such technology as an ultrasonic flaw detection method, eddy-current flaw detection method, magnetic flaw detection method or staining flaw detection method as in the conventional detection.
Further, Japanese Patent Application Publication JP-A-56-12552 (pp. 1-2 and FIG. 1) discloses a crack detection method, by which even an inexperienced person can easily determine the presence or absence of flaw, without requiring a person having experience like the staining flaw detection method. After the application of the liquid and detection, it does not require application and removal of staining penetrating agent and developing agent like the staining flaw detection method, but requires only waiting for natural evaporation of a liquid.
According to the crack detection method, the presence of crack can be detected by impregnating a liquid such as water or ethyl alcohol in the crack, scanning the surface of the object with dried surfaces by using a temperature sensor or an alcohol sensor, and detecting the concentration of the vapor of the liquid remaining within the crack.
However, what is sensed by the sensor is the vapor in a range extremely near the sensor, and therefore, it is necessary to scan the entire object along the object surface in the crack detection method. Consequently, the detection takes much scanning time for a large-scaled object such as a ship or aircraft.